In the quest for hydrocarbon reservoirs, companies employ many data-gathering techniques. The most detailed, albeit localized, data comes from well logging. During the well-drilling process, or shortly thereafter, driller pass logging instruments through the well bore to collect information about the surrounding formations. The information is traditionally collected in “log” form, i.e., a table, chart or graph of measured data values as a function of instrument position. The most sought-after information relates to the location and accessibility of hydrocarbon gases and fluids.
Resistivity, density, and porosity logs have proven to be particularly useful for determining the location of hydrocarbon gases and fluids. These logs are “open hole” logs, i.e., log measurements that are taken before the formation face is sealed with tubular steel casing. The present application focuses on a new way to measure density without reliance on a radioactive source.
Density is traditionally measured by determining the scattering and absorption of gamma rays emitted from a gamma ray source. Traditionally, density logging tools employ Cesium 137, although other sources such as Americium 241 can also be used. Radioactive sources present certain risks to human health and could potentially be a primary ingredient in weapons of terrorism. Even in routine field operations, oilfield workers encounter radiation exposure risks from the use of these sources. When exposed to sufficient radiation from such sources, humans experience cellular damage that can cause cancer and (at higher doses) radiation sickness. These adverse health effects are often lethal. The source materials described above have long half-lives (30 years for cesium 137, and 5300 years for americium 241), meaning that the radiation from these sources will persist for a very long time if they should be accidentally or intentionally dispersed into the environment.
It should come as no surprise, then, to discover that the government heavily regulates the possession and transportation of radioactive sources. See, e.g., 10 CFR Part 1-Part 1060 (regulations from the NRC and DOE) and Federal Register vol. 70, no. 44, Jul. 28, 2005 (Proposed rule changes to 10 CFR Parts 20, 32, and 150, concerning the NRC National Source Tracking Database). Such regulations impose considerable costs for establishing and maintaining compliance. Despite such regulations, the authors are given to understand that on average, at least one such radioactive source is misplaced or stolen each year. See, e.g., Russell Gold and Robert Block, “Radioactive Material Is Stolen From Halliburton”, Mar. 6, 2003 (discussing the theft of a radioactive source and the dangers of a dirty bomb).
In addition, extensive safety procedures are needed to protect workers who transport, store, and use radioactive sources. Radiation from such sources can produce heat, ionization, and chemical changes which lead to corrosion of storage containers. Regular “wipe” tests are conducted to monitor sources for leakage, radiation sensors are put into storage facilities to monitor radiation levels, and employees are given radiation-sensitive badges to monitor employee exposure levels. Cumulatively, the tests, monitoring equipment, transportation, and storage facilities present a severe budgetary impact to any company that employs such sources.
Moreover, when compliance efforts are combined with necessary safety procedures, the result is a considerable effort and delay in getting a radioactive source to the location in the field where it is needed. To further compound the problem, the preferred radioactive sources are in short supply. The largest supplier of americium 241 was the US Department of Energy, which had accumulated a stockpile of this material from various refining operations on other radioactive materials. These stockpiles have now been exhausted, and currently the only continuing source of this material is an aging breeder reactor in eastern Europe.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereof are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the appended claims.